This invention relates generally to preamplification circuits for connection between a sensor and other circuitry and particularly to preamplification circuits for connection between a sensor having a low output impedance and readout circuitry having input impedance greater than the output impedance of the sensor. Still more particularly, this invention relates to preamplification circuits capable of operating at cryogenic temperatures to interface between a sensor such as a low impedance, low noise infrared sensor and a readout multiplexer circuit.
Previous techniques for interfacing sensors in a cryogenic environment have required wires between the sensors and multiplexer circuitry located outside the cryogenic environment. The packaging of such systems is costly and may permit introduction of noise since the signals output from such sensors are ordinarily at low levels. Typical sensors have output impedance in the range of 1 to 10,000 ohms, and typical readout circuitry input impedances are on the order of 10.sup.6 ohms so that impedance matching is essential to transfer the sensor output into the readout circuitry.
Devices such as JFETs achieve low noise operation only when operated at bias currents, and consequently, bias powers that are unsuitable for low temperature applications, especially when a large number of sensor/preamplification channels are operated together. Silicon bipolar devices have insufficient gain at the 77.degree. K. temperature of liquid nitrogen to provide sufficient amplification of the signal output from the sensor. Gallium arsenide transistors have excessive noise at low frequencies for many sensor applications, including those that include HgCdTe detectors, which are commonly used photosensitive infrared detectors. Silicon MOS devices have noise levels that exceed the noise levels of typical sensors used in low temperature measurements.